Programme

I.9 -- Invited, VCTP-49

Date: Wednesday, 31 July 2024

Time: 16:40 - 17:20

First principles and machine learning studies of nanoconfined aqueous solutions in a large pressure-temperature range

Ding Pan

Department of Physics and Department of Chemistry, Hong Kong University of Science and Technology,

This talk presents a study on the impact of nanoconfinement on the physical and chemical properties of aqueous solutions, with a particular focus on the reactions of CO$_2$ in water under extreme pressure-temperature conditions. Carbon storage and transport below Earth's surface have significant implications for the carbon budget in the atmosphere, and underground aqueous solutions are often confined to the nanoscale. However, the molecular-scale chemical speciation and reaction mechanisms of these systems are not yet fully understood. To address this issue, we performed extensive ab initio molecular dynamics (AIMD) simulations to investigate aqueous carbon solutions confined by graphene and stishovite (SiO$_2$) at 10 GPa and 1000 ~ 1400 K. The results show that CO$_2$(aq) reacts more in nanoconfinement than in bulk. These findings suggest that CO$_2$(aq) in deep Earth is more active than previously thought, and confining CO$_2$ and water in nanopores may enhance the efficiency of mineral carbonation. We further constructed Markov state models based on AIMD to elucidate the reaction mechanisms and kinetics of dissolved carbon in supercritical water both in the bulk and nanoconfined states. Unlike many previous molecular simulations using enhanced sampling methods, our method can automatically identify complex reaction coordinates and pathways with multiple intermediates using unsupervised machine learning techniques instead of a priori human speculation. In the bulk solution, CO$_2$ tends to directly react with H$_2$O or OH$^-$ to generate HCO$_3^-$ and H$_2$CO$_3$(aq), whereas under graphene nanoconfinement the dissolution of CO$_2$ involves the pyrocarbonate (C$_2$O$_5^{2-}$(aq)) ion as an intermediate state. While it is known that pyrocarbonate does not exist in aqueous solutions, our study suggests that the extreme hydrophobic confinement may greatly enhance the stability of pyrocarbonate ions in water. Our study provides valuable insights into the reaction kinetic network of aqueous carbon, yielding significant implications for the deep carbon cycle and the sequestration of CO$_2$. Combining quantum molecular dynamics with Markov state models shows great potential to elucidate complex reaction pathways and kinetics. [1] Nore Stolte, Rui Hou, Ding Pan, Nat. Commun. 13, 5932 (2022) [2] Chu Li, Yuan Yao, Ding Pan, arXiv:2401.07019

Presenter: Pan Ding